Energy harvester utilizing external magnetic field

ABSTRACT

Apparatus and method for harvesting energy from the environment and/or other external sources and converting it to useful electrical energy. The harvester does not contain a permanent magnet or other local field source but instead relies on the earth&#39;s magnetic field of another source of a magnetic field that is external to the sensing device. One advantage of these new harvesters is that they can be made smaller and lighter than energy harvesters that contain a magnet and/or an inertial mass.

RELATED APPLICATIONS

This application claims the benefit of priority to U.S. ProvisionalApplication No. 60/758,042 filed Jan. 11, 2006 entitled “Novel EnergyHarvester Without Moving Parts” by J. Huang et al., U.S. ProvisionalApplication No. 60/790,921 filed Apr. 11, 2006 entitled “WirelessTransfer of Electrical Power From Outside a Body to Inside a Body” by J.Huang et al., and U.S. Ser. No. 10/730,355 filed 8 Dec. 2003 entitled“High Sensitivity, Passive Magnetic Field Sensor and Method ofManufacture” by J. Huang et al., which claims priority to U.S.Provisional Application No. 60/431,487 filed 9 Dec. 2002, thedisclosures of which are hereby incorporated by reference in theirentirety.

FIELD OF THE INVENTION

This invention relates to apparatus and methods for harvesting energyfrom the environment and other sources external to the harvester andconverting it to useful electrical energy.

BACKGROUND OF THE INVENTION

Energy harvesters are known that convert vibrational energy intoelectrical energy. The electrical energy produced can then be stored orused by other devices. For example, the vibrations of an airconditioning duct can be converted into electrical energy by an energyharvester and the electrical energy then used to power a sensor thatmeasures the air temperature in the duct. The sensor does not requireelectrical wiring to a remote source of power or periodic batterychanges.

There are a variety of such devices for generating electrical power fromvibrations, oscillations or other mechanical motions. Generally suchdevices are categorized as inductive, capacitive, and/or piezoelectricdevices. While each of the known types of vibrational energy harvestershave different advantages, they also have drawbacks such as: the needfor heavy, powerful permanent magnets (in the case of inductive energyharvesters) to produce a sufficiently large flux density; an auxiliarysource of power, such as a battery (in the case of capacitiveharvesters); the need for large vibration frequencies and/or a heavyinertial mass to generate sufficient vibrational energy for harvesting(for all types of vibration energy harvesters); undesirable levels ofdamping and noise generation from interaction between the locallygenerated magnetic field and nearby metallic parts (for inductiveharvesters); and other disadvantages such as size or weight that makethem unsuitable for use in a remote or generally inaccessible location.

As shown in FIG. 1, a typical magnetic device 2 to harvest vibrationalenergy consists of a permanent magnet 3 (an internal local field source)attached to a housing 7 and connected by a spring 4 to a passivemagnetic field sensor 5 (e.g., an induction coil or a passivemagnetostrictive/electroactive field sensor). External vibrations causea relative motion of the magnet 3 and sensor 5, producing an electricalvoltage across a load 8 and a current through the load. In addition torequiring a local magnetic field source (e.g., a permanent magnetdisposed adjacent to the field sensor), these devices also typicallyinclude an inertial mass 6 (also known as a proof mass) to increase thevibrational energy generated. The inertial mass may be rigidly attachedto the sensor (as shown), may be the same or separate from the permanentmagnet, or may comprise part of the sensor itself. As a furtheralternative, the magnet may comprise part of the moving proof mass, asopposed to being fixed to the housing. In each case, the inertial massand the permanent magnet increase the size and weight of the device.

It would be desirable to provide a magnetic energy harvester that doesnot require a local magnetic field source as part of the device itself.It would also be beneficial to provide a magnetic energy harvester thatdoes not require a source of vibrational or other mechanical motion.

SUMMARY OF THE INVENTION

In accordance with various embodiments of the present invention, anapparatus is provided for harvesting energy from the environment orother remote sources and converting it to useful electrical energy. Theharvester does not contain a permanent magnet or other local fieldsource but instead relies on the earth's magnetic field or anothersource of a magnetic field that is external to the sensing device. Oneadvantage of these new harvesters is that they can be made smaller andlighter than energy harvesters that contain a magnet. Another advantageis that they do not require vibrational energy to function.

According to various embodiments of the invention disclosed herein, theharvester differs from those of the prior art by the absence of apermanent magnet or other local (internal) field source. In these newdevices, a change in the state of magnetization of the sensing elementmay be achieved in one (or both) of two general ways:

-   -   1. The magnetic flux density in the sensing element may be        altered by changes in the orientation of the sensor (movement of        the sensor) with respect to a static (non-changing) external        field. For example, the magnetization vector M of the sensing        element may rotate due to changes in the orientation of the        sensing element with respect to the earth's field. Such movement        of the sensing element may be achieved by attaching the sensing        element to a piece of rotating machinery or a rotating part on a        vehicle. Alternatively, the sensing element may be suspended on        its axis and allowed to rotate (due to external vibrations), the        rotation causing a change in its orientation in the earth's        field.    -   2. Alternatively, the sensing element may remain stationary and        be operated on by a remote changing magnetic field from any of a        variety of sources. The remote changing magnetic field can be        produced by an electrical transformer, motor, electronic device,        moving machinery or inductive wire or coil which is relatively        remote (acting at a distance or through a non-magnetic barrier)        on the sensing element. The changing (e.g., alternating)        magnetic field source can be designed to couple with a remote        sensing element efficiently in terms of frequency, distance,        field orientation and magnitude to deliver power remotely to the        sensing element.

When operating from the earth's magnetic field, the power harvested maybe less than that achieved with a prior art vibrational energy harvesterhaving a built-in magnetic field. The power harvested from a remotefield source will be measured in microwatts per centimeter cubed(μW/cm³), as opposed to milliwatts per centimeter cubed (mW/cm³) forenergy harvesters that include a strong magnet. However, for certainapplications the power delivered by such a small, lightweight and simpleenergy harvester will be sufficient and enable new applications.

When operating near some man-made external sources of alternatingmagnetic fields, or when such an external field source is brought nearto the energy harvester, the power harvested can be considerably greaterbecause the strength of such fields is often considerably greater thanthe earth's magnetic field (H_(earth) is approximately 0.3 Oe or 3micro-Tesla).

A preferred sensing element for use in the present invention is based ona class of passive magnetostrictive electroactive (PME) magnetic fieldsensors that produce a voltage when exposed to a changing magneticfield. The sensing element is preferably a layered structure (e.g.,sandwich) of magnetostrictive material bonded to an electroactivematerial, the latter being poled in a direction preferably parallel tothe plane of the magnetostrictive layer(s). An external magnetic fieldcauses a magnetization change in the magnetostrictive layer(s), whichrespond(s) with a magnetoelastic stress. Part of the stress istransferred to the electroactive layer that responds by producing avoltage given by V_(i)=g_(ij)σ_(j)L_(i). Here, L_(i) is the distancebetween the electrodes across which the voltage V_(i) is measured, σ_(j)is the stress transferred to the electroactive component, and g_(ij) isthe stress-voltage coupling coefficient. The voltage is greatest whenthe direction i=j. However, in different applications the principalstress and induced voltage may lie in orthogonal directions (e.g., 1-3operation), or the principal stress and voltage may act along differentaxes (e.g., 1-5 operation).

The energy harvester of the present invention is more than a simplepassive magnetostrictive/electroactive (PME) field sensor. A simple PMEfield sensor is comprised of materials and dimensions designedpreferably to produce a large voltage across a high impedance circuit,the voltage being indicative of the field of interest. The electroniccircuit for a simple PME sensor is designed to register a field value.In contrast, the PME energy harvester of the present invention iscomprised of materials and dimensions designed preferably to produce avoltage and current that match the impedance of the load to be driven.The PME energy harvester is coupled to an electronic circuit thatconverts the PME output to power for immediate use or storage. The PMEelement is preferably optimized to respond to the field strength of theintended environment, which would generally be much greater than that ofa pure field sensor.

This new type of energy harvester can be simpler, lighter and/or morecompact than those requiring a permanent magnet as a field source, andalso those requiring an inertial mass for enhancing vibrational energy.For example, suitable applications may include wireless monitoringapplications, wherein wireless monitoring is meant to include selfpowered sensing of local conditions and processing of the sensor outputand self powered wireless communication to a central data processingpoint. Other suitable applications might include wireless transfer ofelectrical power over a small distance to a location inaccessible viaelectrical leads or not convenient for battery replacement. Morespecifically, these applications may include supplying power for:

-   -   wireless health monitoring or condition based maintenance;    -   supplementing power or recharging batteries without physically        accessing them;    -   elimination of wiring of electrical devices remote from a power        source;    -   wireless monitoring of temperature, airflow, humidity and gas        content in heating, ventilation and air conditioning (HVAC)        systems;    -   wireless monitoring of traffic flow, turbulence, noise or        personnel movement;    -   wireless, self powered security systems;    -   powering of mobile electronic instruments;    -   passive detection of creep or crack propagation in structures        for condition based maintenance; and    -   powering of devices implanted in a living body (or to another        inaccessible location) for purposes of sensing, transmitting, or        actuating (e.g., motors, pumps, switches, valves, electrodes),        as well as for accomplishing therapy or other functions that        require a voltage and/or current.

Furthermore, applications of the new energy harvester are not limited tovibration rich environments. A simple repetitive motion or rotation ofthe more sensitive PME devices as described herein allows them tooperate from the earth's magnetic field. They can be placed inphysically inaccessible locations and activated from a remote fieldsource that is either present in the environment or placed there for thepurpose of energizing the device.

In one embodiment of the invention: an energy harvester without a localfield source comprises:

a magnetic field sensing element including one or more layers ofmagnetostrictive material having a magnetization vector that responds tovariations in an applied magnetic field by generating a stress, and oneor more layers of electroactive material, mechanically bonded to thelayer of magnetostrictive material, that responds to the stress bygenerating a voltage; and

a circuit coupled to the sensing element that converts the voltage toelectrical power for immediate use or storage, wherein the sensingelement either:

-   -   a) moves relative to a remote static external magnetic field,        such that changes in orientation of the sensing element with        respect to the external field generates the voltage; or    -   b) is stationary with respect to a remote changing external        magnetic field, wherein the changing external field causes the        sensing element to generate the voltage.

In various embodiments:

the electrical power comprises a voltage and current suitable for anintended application;

the magnetostrictive material layer has a magnetization vector thatresponds to variations in the magnetic field by rotating in a plane andwherein the electroactive material is poled in a direction substantiallyparallel to the plane in which the magnetization vector rotates;

the sensing element is mounted to an object that moves relative to theapplied magnetic field;

the variations in the applied external field are in one or more ofmagnitude and direction of the field;

the sensing element is mounted such that local vibration changes itsorientation with respect to the applied magnetic field;

the sensing element includes electrodes for measuring the voltagegenerated and wherein the electrodes are configured such that thedistance between the electrodes and cross sectional area between theelectrodes are tailored to produce a desired electrical power;

the magnetization vector rotates relative to the electrode axis due tochanges in the orientation of the sensor in the applied external field;

local vibrations also change the orientation of the sensor with respectto the applied external field;

the remote magnetic field is generated by one or more of an electricaltransformer, motor, actuator, switch, electronic device, movingmachinery or inductor;

the inductor is a wire or coil through which an alternating current isflowing, to produce the remote changing external magnetic field;

the sensing element is rigidly attached to an inertial mass;

the sensing element includes an inertial mass;

the changing external field or sensing element movement is at vibrationor power transmission frequencies of no greater than 1 kHz;

the changing external field is at a resonance frequency in the range ofthat of the sensing element;

the changing external field is in a range of 20 to 50 kHz;

the external field frequency is equal to or close to the resonancefrequency of the sensor, which varies roughly according to the equation

${fr} \approx {\frac{1}{2L}\sqrt{\frac{E_{eff}}{\rho_{eff}}}}$where L is a characteristic length of the sensor and E_(eff) and ρ_(eff)are the elastic modulus and mass density appropriate to describe thecomposite magnetostrictive/electroactive sensor properties;

the changing external field and sensing element are within a resonantfrequency range;

the circuit is within the resonant frequency range;

the external changing field is outside a human or other animal body andthe sensing element is inside the body.

In another embodiment of the invention, a method of harvesting energycomprises:

providing a magnetic field sensing element including one or more layersof magnetostrictive material having a magnetization vector that respondsto variations in an applied magnetic field by generating a stress, andone or more layers of an electroactive material, mechanically bonded tothe layer of magnetostrictive material, that responds to the stress bygenerating a voltage;

wherein the voltage is generated by either:

moving the sensing element relative to a remote static external magneticfield, such that changes in orientation in the sensing element withrespect to the external field generates the voltage; or

the sensing element is stationary with respect to a remote changingexternal magnetic field, and the changing external field causes thesensing element to generate the voltage; and

converting the generated voltage to electrical power for immediate useor storage.

These and other advantages of the present invention may be betterunderstood by referring to the following detailed description andaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block schematic diagram of a prior art energy harvesterdevice having a permanent magnet and inertial mass;

FIG. 2 is a schematic diagram of an energy harvester according to oneembodiment of the invention, in which the harvester is attached to amoving part and is actuated by its motion in the earth's magnetic field;

FIG. 3 is a schematic diagram of another embodiment of the energyharvester of the invention, in which a stationary harvester is actuatedby a changing external field (H_(earth-modified)), the external fieldbeing that of the earth's field changed in magnitude or direction by amoving object in its path;

FIG. 4 is a schematic diagram of another embodiment of the invention inwhich a stationary harvester device is activated by a remote externalchanging magnetic field H_(ext), where H_(ext) is due to a permanentmagnet affixed to a moving part exterior to the harvester;

FIG. 5 is a schematic diagram of one construction of a sensing elementuseful in the present invention;

FIG. 6 is a schematic diagram of a circuit for converting and changingelectrical output of the harvester to a DC electrical signal,conditioning, storing and providing the resulting electrical energy to aload;

FIG. 7 is a graph of PME voltage output versus field at 20 Hz for oneembodiment of the invention;

FIGS. 8 a-8 b are schematic diagrams of two embodiments of an energyharvester wherein an external changing magnetic field generated by acoil is transmitted through tissue to an embedded PME sensor;

FIG. 9 is a graph of PME power versus load at 1 Oe and 29 kHz accordingto another embodiment of the invention; and

FIG. 10 is a block schematic diagram of a passivemagnetostrictive/electroactive sensor constructed in accordance with oneembodiment of the invention.

DETAILED DESCRIPTION

FIG. 2 illustrates a first embodiment of an energy harvester deviceaccording to the invention, wherein a static external magnetic field,here H_(earth) (the earth's magnetic field), acts at a distance (orthrough a non-magnetic barrier) on a sensor whose orientation in thefield changes with time. Thus, the flux density in the sensor is alteredby changes in the physical orientation of the sensor with respect to thedirection of the earth's field (or other substantially static field).FIG. 2 illustrates this with a non-magnetic barrier 14 separating theenergy harvester 12 on the left, attached to a moving object (rotatingmachinery part 16), from the source of external field 18 on the right.The arrow 26 extending across the width of sensor 12 represents theplane of the magnetization vector M. The arrow 20 illustrates rotationof sensor 12 from a first position 22 (labeled X) to a second position24 (labeled X′). Alternatively, the sensor can be attached to a rotatingpart on a vehicle, a door, or other object that moves relative to theearth's field. As a further alternative, the sensor may also besuspended on its axis such that linear vibration acting on the sensorchanges its orientation in the earth's field. The change in sensororientation requires some asymmetry in the suspension for the vibrationsto cause rotation of sensor about its axis. It is preferable that thechange in sensor orientation be such that the axis between itselectrodes changes orientation relative to the static field direction.In other words, the sensor motion should preferably not be rotationabout the axis of the applied field.

A second embodiment of the invention is shown in FIG. 3, wherein achanging external field, H_(earth-modified), acts at a distance (orthrough a non-magnetic barrier) on a stationary sensor 32. Here theorientation of the field at the sensor location changes with time. InFIG. 3, a moving magnetic object (rotating disk 34) in the path of theearth's ambient field causes a change in that field, and this changingfield (H_(earth-modified)) then reaches (acts on) the static sensor 32.

In a further alternative embodiment, shown in FIG. 4, a permanent magnet42 is affixed to a rotating or moving object 44 remote from the sensor48. The change in position of the magnet 42 relative to the stationarysensor 48 causes a changing field (see field lines 46 of H_(ext)) thatacts on the static sensor 48. Arrow 45 illustrates rotation of themagnet to a second position (shaded area 42′ at the bottom of rotatingdisk 44) and the changing field as dashed lines 46′. Other sources ofchanging or alternating magnetic field can be found near electricaltransformers, motors, actuators, switches, many electronic devices,inductor wires or coils, and near areas of high vehicle traffic ormoving machinery. The remote changing magnetic field source can bedesigned to couple with the sensor efficiently in terms of frequency,distance and magnitude to deliver power remotely to the sensor.

FIG. 5 is a schematic illustration of a preferred sensor configuration50 for use in the present invention. In this embodiment, a central layerof an electroactive (e.g., ceramic, polymer or single crystalpiezoelectric; or a relaxor ferroelectric) material having apolarization vector P is shown, sandwiched between two layers 54, 56 ofmagnetostrictive material (e.g., of a soft ferromagnetic material havinga non-zero magnetostriction) on opposing faces of central layer 52. Eachmagnetostrictive layer has a magnetization vector M which is caused torotate in the plane of the magnetostrictive layer by an applied field H.A pair of electrodes 58 are disposed at opposite ends of thepiezoelectric, the axis between the electrodes being parallel to theplane in which the magnetization vectors rotate. The voltage V generatedin the piezoelectric, resulting from the magnetoelastic stress generatedin the magnetic layers and transferred to the piezoelectric, can bemeasured in a circuit coupled to the electrodes attached to thepiezoelectric layers.

The materials and configuration of the sensor may vary depending uponthe particular application. While it is generally desirable to use amagnetic material with large magnetostriction for the magnetic layer(s),it is generally more important (for optimum power delivery) that themagnetostrictive material have a large product of a magnetostrictivestress and stiffness modulus (see “Novel Sensors Based onMagnetostrictive/Piezoelectric Lamination,” J. K. Huang, D. Bono and R.C. O'Handley, Sensors and Actuators 2006). This insures that themagnetic layer(s) more effectively transfer stress to the piezoelectricmaterial. For example, while FeCo(Hyperco) shows a relatively largemagnetostriction (approaching 100 ppm) and is extremely stiff, theproduct of these parameters translates to a magnetostrictive stress of1.2 MPa. A high-magnetostriction material such as Fe₂(Dy_(2/3)Tb_(1/3))(known as Terfenol-D) on the other hand, is mechanically softer thanFeCo but shows a much larger magnetostrictive strain and itsmagnetostrictive stress approaches 6 MPa.

It is also important (for optimum power generation) that themagnetostrictive stress changes by the largest possible amount under theinfluence of the changing field strength available at the sensor. Forexample, the magnetization vector of FeCo can be rotated in a field of afew tens of Oe (Oersteds) while the magnetization vector of Terfenol-Dcan be rotated in a field of several hundreds of Oe, provided in eachcase they are properly annealed and the aspect ratio of the material inthe magnetizing direction is favorable.

The class of magnetostrictive materials that can be magnetized in theweakest fields consist of a variety of amorphous alloys based on iron(Fe) (optionally in combination with nickel (Ni)) and with glass formerssuch as boron (B) (optionally with silicon (Si)).

Electro-active materials, such as the commercially availablepiezoelectric lead-zirconate-titanates (PZT) have stress-voltagecoefficients, g₁₃ and g₃₃, with values approximately equal to 10 and 24mV/(Pa-m), respectively. Thus, a stress applied to the piezoelectricparallel to the direction across which the voltage is measured is moreeffective in generating a voltage than a stress transverse to thisdirection (out of the plane in which the vector is rotated by thefield). Further, relaxor ferroelectrics have g_(ij) values that can bethree to four times those of piezoelectrics. Also useful in theseapplications are piezo fibers or manufactured piezo fiber composites.They may have interdigitated electrodes with various spacings to produceelectric fields along the piezo fibers or they may be electroded acrossthe thickness of the fibers. Polymeric piezoelectric material(s) (e.g.,poly-vinylidene-difluoride PVDF) may be advantageous in someapplications.

There are a number of ways to increase the strength of the earth's fieldentering the magnetostrictive layers so as to enhance the powerharvested. One way is to use flux concentrators (e.g., fan-shaped softmagnetic layers) placed in series with the sensing element in thepresence of the field.

The sensor output can be adapted for immediate use or storage bycoupling the sensor to an electronic circuit. One such circuit 70 isshown in FIG. 6. On the left hand side, a PME energy harvester Y1 isshown. A diode bridge D1 is disposed in parallel across the harvesteroutput. The full wave diode bridge converts the AC electric charge onthe harvester to a DC charge. Connected in parallel to the diode bridgeis an energy storage capacitor C1 which stores the harvested energy as avoltage across it. Parallel to the capacitor is a limiter zener diode D2which prevents overcharging of the capacitor C1 beyond its breakdownvoltage. Next provided in parallel to the capacitor and diode bridge isa voltage regulator U1. The voltage regulating circuit reduces thecapacitor voltage to a useful level for a load. The voltage regulatedoutput across J1 is applied to the load, here represented as a loadimpedance Z1, which typically includes resistive and capacitiveelements, and which uses the harvested energy to do useful work.

FIG. 7 is a comparison of the PME output voltage signal (RMS voltage inmillivolts) versus magnetic field strength (H in telsa). In thisembodiment the changing external magnetic field is at a low frequency of20 Hz. The PME voltage output linearly increases from 0 to 650millivolts with increasing magnetic field strength from 0 to 20e.Alternatively, the magnetic field can be static and the position of thePME varying. The substantially linear relationship between the PMEvoltage output and magnetic filed strength is representative for lowfrequency applications (where the field changes or the sensor motionsare at low vibration frequencies or power transmission frequencies).

Alternatively, a higher frequency external field can be used to obtain agreater level of power from the PME (compared to the low frequencyoperation of FIG. 7). This is illustrated with the embodiment andresulting power output shown in FIGS. 8-9. FIG. 8 a illustrates a meansof delivering power inside a living organism (or any inaccessible ordifficult to access location) without requiring the use of electricalwiring between the source of the power and the target device and withoutrequiring (or with diminished need for) batteries. FIG. 8 a shows anexternal loop antenna 80 generating an alternating magnetic fieldoutside of the body. The magnetic field 84 generated by this loopantenna is transmitted through the skin and other tissue 82 to anembedded PME sensor 86 producing a resulting output voltage V. The powertransmission here is achieved not by a high frequency microwave, RF orother electromagnetic wave, but rather by means of a relatively lowfrequency, benign, alternating magnetic field. Microwaves and otherelectromagnetic waves having a wavelength comparable to or less than thedistance between the source and receiver, are rapidly attenuated bywater or metals, and thus would not be suitable in this application.Instead, the loop antenna produces a low-frequency, magnetic-rich waveswhich are left essentially unattenuated by tissue (assuming nointervening magnetic material), and which do not have problems withtissue heating that accompanies microwaves. Alternatively, instead of aloop antenna (with no core) the external source could be a core-filledcoil antenna such as a solenoid coil with core 90 (FIG. 8 b), whereinthe core may significantly enhance the field 92 in the body

The field generated by a loop, solenoid or core-filled coil antenna isricher in magnetic field strength than electric field strength within arange comparable to the wavelength of the radiation. The wavelength isgiven by the equation λ=c/f, where c is the speed of light in themedium, and f is the frequency of the radiation. At 1 MHZ (megahertz) inair, λ equals 300 m (meters); at 100 MHZ, λ equals 3 m. Thus, there is awide range of frequencies over which to transmit a magnetic-field richelectromagnetic wave without significant attenuation.

The implanted passive magnetostrictive/electroactive (PME)sensor/transducer 86 receives and converts the AC magnetic field 84 toan AC voltage that can be processed to produce power needed for aparticular application. For example, this apparatus can be used inpowering internal pumps, sensors, valves and transponders in human andanimals. More generally, it can be used to power devices which monitorhealth, organ function or medication needs, and for performing activefunctions such as pumping, valving, stimulation of cell growth oraccelerated drug or radiation treatment locally. The described means ofdelivering power inside a living organism can be achieved without theuse of electrical wires in between the source of the power and thetarget device and without the need, or diminished need, for batteries.

The wireless power harvested for the remote application can beoptimized, for example, if resonance is achieved at each stage oftransduction. Thus the external power source and the transmit antennashould be in resonance. The PME device should also be in resonance withthe field it responds to, and the PME device should also be in resonancewith the part of the circuit that receives the signal from the PMEdevice. By careful design and material selection, it is possible for allthree resonances to closely coincide. FIG. 9 illustrates one example ofa PME power output (mW/cm³) versus load (ohm) for one such resonantsystem operating at a frequency of 29 kHz and a field of one (1) Oe.

The remote sensor can be used not only for powering internal pumps,sensors and transponders in humans and animals, but can be used tomonitor the flow of things (people or inanimate objects) past gates(either for security or tracking purposed).

There will now be described in more detail alternative sensorconfigurations and sensor materials which may be useful in variousembodiments or the present invention.

FIG. 10 is a block schematic diagram of one embodiment of a passivemagnetostrictive sensor useful in the practice of the invention. Thesensor 400 comprises a magnetic layer 402 that is bonded to apiezoelectric layer 404 by a suitable non-conductive means, such asnon-conductive epoxy glue. Although only one magnetic layer 402 is shownbonded to a single piezoelectric layer 404, those skilled in the artwould understand that two or more magnetic layers can be used. Themagnetization vector 415 (M) of the magnetic material 402 rotates in theplane 416 of the magnetic layer 402 when an external magnetic field (H)is applied as shown by the arrow 414. The rotation of the magnetizationvector M causes a stress in the magnetostrictive layer 402 which is, inturn, applied to the piezoelectric layer 404 to which the magnetic layer402 is bonded. In this design the direction of magnetization, M, rotatesin the preferred plane of magnetization, changing direction from beingparallel to perpendicular (or vice versa) to a line joining theelectrodes. This maximizes the stress change transferred to theelectroactive element.

The stress-induced voltage in the piezoelectric material 404 is measuredacross a pair of electrodes 406 and 407 of which only electrode 406 isshown in FIG. 10. The magnitude of the voltage developed acrosselectrodes 406 and 407 is a function of the magnetic field strength forH<H_(a), the anisotropy field (at which M is parallel to the appliedfield) and can be utilized to power a device 410 that is connected toelectrodes 406 and 407 by conductors 412 and 408, respectively.

The sensor is constructed so that stress-induced voltage is measured ina direction that is parallel to the plane 416 in which the magnetizationrotates. The stress is generated in the magnetic material 402, whichresponds to an external magnetic field 414 (H) with a magnetoelasticstress, σ_(mag), that has a value in the approximate range of 10 to 60MPa. Because the magnetic material 402 is bonded to a piezoelectriclayer 404, the layer 404 responds to the magnetostrictive stress with avoltage proportional to the stress, σ_(mag), transmitted to it.Piezoelectric materials respond to a stress with a voltage, V, that is afunction of the applied stress, a voltage-stress constant, and thedistance, l between the electrodes. In particular,

δ V = g_(ij)^(piezo)f δσ_(mag)l

Here δσ_(mag) is the change in magnetic stress that is generated in themagnetic material by the field-induced change in its magnetizationdirection. A fraction, f, of this stress is transferred to theelectroactive element. δV is the resulting stress-induced change involtage across the electrodes on the electroactive element.

If the voltage is measured in a direction orthogonal to the direction inwhich the stress changes, then g_(ij)=g₁₃. As mentioned previously,typically piezoelectric values for g₁₃ are 10 millivolt/(meter-Pa).However, if the voltage is measured in a direction parallel to theprincipal direction in which the stress changes in accordance with theembodiment of FIG. 4, then g_(ij)=g₃₃. Thus, the sensor operates in ag₃₃ or d₃₃ mode. For a typical piezoelectric material g₃₃=24millivolt/(meter-Pa)=0.024 volt-meter/Newton. In this case, a stress of1 MPa generates an electric field of 24 kilovolt/meter. This fieldgenerates a voltage of 240 V across a 1 cm (l=0.01 m) wide piezoelectriclayer.

The stress generated by the magnetic material 402 depends on the extentof rotation of its magnetization, a 90 degree rotation producing thefull magnetoelastic stress. The extent of the rotation, in turn, dependsof the angle between the magnetization vector 415 and the appliedmagnetic field direction 414 and also depends on the strength of themagnetic field and on the strength of the magnetic anisotropy(magnetocrystalline, shape and stress-induced) in the magnetic layer.The fraction, f, of the magnetostrictive stress, σ_(mag), transferredfrom magnetic to the piezoelectric layer depends on the(stiffness×thickness) product of the magnetic material, the effectivemechanical impedance of the bond between the magnetic and electricelements (proportional to its stiffness/thickness), and the inverse ofthe (stiffness×thickness) of the piezoelectric layer.

A quality factor may be defined from the above equation to indicate thesensitivity of the device, that is, the voltage output per unit magneticfield, H (Volts-m/A):

$\frac{\partial V}{\partial H} = {g_{33}^{piezo}{f\left( \frac{\partial\sigma_{mag}}{\partial H} \right)}l}$

The characteristics of a suitable magnetostrictive material arepreferably large internal magnetic stress change as the magnetizationdirection is changed. This stress is governed by the magnetoelasticcoupling coefficient, B₁, which, in an unconstrained sample, producesthe magnetostrictive strain or magnetostriction, λ, proportional to B₁and inversely proportional to the elastic modulus of the material. It isalso important that the magnetization direction of the magnetic materialcan be rotated by a magnetic field of magnitude comparable to theapplied field. In general, the magnetic material should also bemechanically robust, relatively stable (not prone to corrosion ordecomposition), and receptive to adhesives. In addition, if the magneticmaterial is electrically non-conducting, it can be bonded to theelectroactive element with the thinnest non-conducting adhesive layerthat provides the needed strength without danger of shorting out thestress-induced voltage developed across the electroactive element. ForPME devices in which the voltage is measured across electrodes that arenot the same as the megnetostrictive layers, care must be taken that themagnetostrictive layers not short out the voltage between the measuringelectrodes. This can be accomplished by using a non-conducting adhesiveto insulate the magneostrictive layer(s) from the electroactiveelement(s).

Many known magnetostrictive materials can be used for the magnetic layer402. These include various magnetic alloys, such as amorphous-FeBSi orFe—Co—B—Si alloys, as well as polycrystalline nickel, iron-nickelalloys, or iron-cobalt alloys such as Fe₅₀Co₅₀ (Hyperco). For example,amorphous iron and/or nickel boron-silicon alloys of the formFe_(x)B_(y)Si_(1-x-y), where 70<x<86 at %, 2<y<20, and 0<z=1-x-y<8 at %are suitable for use with the invention with a preferred compositionnear Fe₇₈B₂₀Si₂. Also suitable are alloys of the formFe_(x)Co_(y)B_(z)Si_(1-x-y-z) where 70<x+y<86 at % and y is between 1and 46 at %, 2<z<18, and 0<1-x-y-z<16 at %, with a preferred compositionnear Fe₆₈Co₁₀B₁₈Si₄. Iron-nickel alloys with Ni between 40 and 70 at %with a preferred composition near 50% Ni can be used. Similarly,iron-cobalt alloys with Co between 30 and 80% and a preferredcomposition near 55% Co (such as Fe₅₀Co₅₀.) are also suitable.

Another magnetostrictive material that is also suitable for use with theinvention is Terfenol-D® (Tb_(x)Dy_(1-x)Fe_(y)), an alloy of rare earthelements Dysprosium and Terbium with the transition metal iron,manufactured by ETREMA Products, Inc., 2500 N. Loop Drive, Ames, Iowa50010, among others. Terfenol-D® can generate a maximum stress on theorder of 60 MPa for a 90-degree rotation of its magnetization. Such arotation can be accomplished by an external applied magnetic field onthe order of 400 to 1000 Oersteds (Oe). Also useful are highlymagnetostrictive alloys such as Galfenol®, Fe_(1-x)Ga_(x). (ETREMAProducts). Softer magnetic materials, such as certain Fe-rich amorphousalloys mentioned above, may achieve full rotation of magnetization infields of order 10 Oe, making them suitable for the magnetic layer in asensor for sensing weaker fields. Finally, it is possible to use certainso-called nanocrystalline magnetic materials. In these polycrystallinematerials, it is generally that case that the magnetization can berotated as easily as it can be in amorphous materials. Butnanocrystalline materials can sometimes be engineered to have largermagnetoelastic coupling coefficients than amorphous materials.

The preferred characteristics of a suitable electroactive layer for thesensor devices are primarily that they have a large stress-voltagecoupling coefficient, g₃₃. In addition, they preferably should bemechanically robust, receptive to adhesives, not degrade the metallicelectrodes that must be placed on them (this is most often easilyachieved when the electrodes are made of noble metals, such as silver orgold). Generally, the electroactive material is chosen on the basis ofhaving a value of g_(ij) greater than 10 mV/(Pa-m).

The electroactive layer can be a ceramic piezoelectric material such aslead zirconate titanate Pb(Zr_(x)Ti_(1-x))O₃, or variations thereof,aluminum nitride (AlN) or simply quartz, SiO_(x). In some applications asingle crystal (as opposed to a ceramic or polycrystalline)piezoelectric material may be advantageous. Alternatively, a polymericpiezoelectric material such as polyvinylidene difluoride (PVDF) would besuitable for applications where the stress transferred from themagnetostrictive material is relatively weak. The softness of thepolymer will allow it to be strained significantly under weaker appliedstress to produce a useful polarization, or voltage across itselectrodes. It is also advantageous in some applications to use anotherelectroactive material, such as an electrostrictive material (forexample, (Bi_(0.5)Na_(0.5))_(1-x)Ba_(x)Zr_(y)Ti_(1-y)O₃) or a relaxorferroelectric material (for example, Pb(Mg_(1/3)Nb_(2/3))₃).Collectively, the piezoelectric, ferroelectric, electrostrictive andrelaxor ferroelectric layers are called “electroactive” layers.

Piezoelectric materials typically have g₃₃˜4×g₃₁ and g₃₃≈20 to 30mV/(Pa-m) which is about 10×d₃₁. For PVDF, g₃₃≈100 mV/(Pa-m) and somerelaxor ferroelectrics can have g₂₂≈60 my/(Pa-m).

Model predictions and experimental results shown in Table 1 compares theparameters g_(ij), in mV/m-Pa, the electrode spacing l in meters, themaximum stress per unit field (B₁/μ_(o)H_(a)) in Pa/T, and calculatedfield sensitivity in nV/nT and the observed field sensitivity, dV/dB.The values tabulated for a g₃₃ device using a relaxor ferroelectric arebased on the data observed with a piezoelectric based sensor and using aratio of g₃₃ for typical relaxors/piezoelectrics.

TABLE 1 max. Sensitivity g_(ij) l stress Calc. Obs. Piezo/magneticsensors: d₃₁ sensor 11 10⁻³ 10⁸ 10⁴   280 d₃₁ sensor 11 10⁻³ 10⁸ 10⁴1,200 d₃₃ sensor 24 10⁻² 10⁹ 2 × 10⁵ 1.5 × 10⁴ Relaxor/magnetic sensors:d₃₃ relaxor/mag sensor 60 10⁻² 10⁹ 10⁶ (10⁵)

The calculated sensitivity in the table is defined with perfect stresscoupling, namely f=1 in MKS units (V/Tesla) as

$\frac{\partial V}{\mu_{0}{\partial H}} \approx {g_{33}^{piezo}l\frac{B_{1}}{\mu_{o}H_{a}}}$

Here B₁ is the magneoelastic coupling coefficient, a material constantthat generates the magnetic stress in the magnetostrictive material,σ_(m), which was used in earlier equations.

Other useful sensor embodiments are disclosed in U.S. Ser. No.10/730,355 filed 8 Dec. 2003 entitled “High Sensitivity, PassiveMagnetic Field Sensor and Method of Manufacture,” by J. Huang, et al.,the subject matter of which is incorporated by reference herein in itsentirety.

Although exemplary embodiments of the invention have been disclosed, itwill be apparent to those skilled in the art that various changes andmodifications can be made which will achieve all or some of theadvantages of the invention.

1. An energy harvester without a local field source comprising: amagnetic field sensing element including one or more layers ofmagnetostrictive material having a magnetization vector that responds tovariations in an applied magnetic field by generating a stress, and oneor more layers of electroactive material, mechanically bonded to thelayer of magnetostrictive material, that responds to the stress bygenerating a voltage; and a circuit coupled to the sensing element thatconverts the voltage to electrical power for immediate use or storage;wherein the sensing element is mounted to an object that moves relativeto the applied magnetic field, such that changes in orientation of thesensing element with respect to the applied field generates the voltage.2. The energy harvester of claim 1, wherein the sensing element includeselectrodes for measuring the voltage generated and wherein theelectrodes are configured such that the distance between the electrodesand cross sectional area between the electrodes are tailored to producea desired electrical power; and wherein the magnetization vector rotatesrelative to the electrode axis due to changes in the orientation of thesensor in the applied external field.
 3. The energy harvester of claim2, wherein local vibrations also change the orientation of the sensorwith respect to the applied external field.